Thermal shielding is used in low pressure steam turbines to reduce through-wall and axial thermal gradients, associated thermal stresses and distortion, and to reduce heat losses to a condenser coupled to receive exhaust steam from the turbine. Such shielding is usually applied to an exterior surface of a turbine inner cylinder. Typically, such shielding covers only a cylindrical outer surface portion of the inner cylinder leaving flanges, bolting members, and the inner cylinder end walls exposed to the external steam environment, i.e., to low temperature steam in the turbine between the inner and outer cylinders. Thermal shielding is generally attached to the inner cylinder cylindrical surface portion by welding studs which are welded to the outer surface of the inner cylinder. Preferably, the welding studs have a tapped hole for receiving attaching screws passing through corresponding holes in the thermal shield. The shielding is generally a light gage of carbon steel, usually about 0.12 inches (3.048 mm) thick. The shielding is produced in many pieces or segments to allow fitting about complex geometries and to provide accessibility to inner cylinder bolting access covers.
Numerous problems may be possible with the above described design. If joints between segments of the thermal shield are not supported by the studs, pieces or sections of shielding may vibrate. Vibration may lead to cracking and structural failures in which part of the thermal shield may break loose causing consequential damage to the turbine condenser. Gaps between segments are sometimes sealed by welding narrow strips of material to the segment at one side of the gap. Due to vibration, erosion, and less than ideal weldability, these sealing strips may become detached, reducing the effectiveness of the thermal shield. Additional failures may result from loss of attachment screws The carbon steel plate of the shield is subject to corrosion and erosion due to the wet steam environment and this effect may be aggravated in turbine units with above average air inleakage leading to accelerated deterioration and thinning of the thermal shield plate.
Incomplete coverage of the inner cylinder outer surface, and poor sealing caused by gaps between the thermal shield and adjacent structures cause the effectiveness of the thermal shield to be diminished. The temperature gradients across the inner cylinder are therefore not reduced as much as desired and this may lead to excessive cylinder distortion and ovality, which may cause blade and seal rubs, and high stress levels in the cylinder structure and bolting members. This, in turn, may cause cylinder structural problems such as cracking of cylinder components, loss of thermal performance due to seal rubs at distorted cylinder sections, and leakage at highly stressed bolted joints. It may impair the thermal performance of the turbine by allowing excessive heat loss from the hot inner cylinder walls to the cold exhaust steam.